1. Field of the Invention
The present invention relates to an automatic temperature control system for a vehicle air conditioner of the air-mix type, which has a low noise level and adequately counteract temperature changes within the interior of the vehicle.
2. Relevant Art
The air-mix type vehicle air conditioner generally comprises a cool air supply mechanism having an air flow control device, an air flow dividing device for dividing a cool air flow supplied from the cool air supply mechanism into a primary flow and a secondary flow, a heat source for heating the primary flow, and a mixing mechanism for mixing the primary flow, as warmed past the heat source, and the secondary flow, which remains cool, with each other.
Referring now to FIG. 1, there is schematically shown a typical example of such type of vehicle air conditioner 1, which is constituted of a cool air supply mechanism 2 having an air supply fan 6 functioning as an air flow control device for supplying through a cooling evaporator 8 an air flow introduced from either outside or inside the interior of a vehicle by a change-over operation of a damper 7, an air flow dividing device 3 such as a butterfly type air-mix damper for dividing a cool air flow supplied from the cool air supply mechanism 2 into a primary flow directed through a heating passage 9 and a secondary flow directed through a by-pass duct 10. Also provided is a heat source 4 such as a heat exchanger utilizing engine heat for heating the primary flow, and a mixing mechanism 5 for mixing the primary flow, as warmed past the heat source 4, and the secondary flow which remains cool, with each other. In this manner, flow conditioned at an adequate temperature is supplied into the vehicle room.
For such a vehicle air conditioner there has been adopted an automatic temperature control system including a circuit for detecting for output as a voltage level an air temperature within the interior of a vehicle, a circuit for detecting for output as a voltage level an air temperature outside the interior of the vehicle, and a circuit interlocked with the foregoing air flow dividing device for output as a voltage level the flow-dividing ratio thereof, in which a signal resulting from a comparison of the sum of those outputs with a reference voltage is used to control the foregoing air flow control device and the air flow dividing device interlocked therewith.
Referring now to FIG. 2, there is shown as an example such a conventional automatic temperature control system, in which an outside air temperature sensor 40, a vehicle room temperature sensor 41 and a potentiometer 42 interlocked with an air-mix damper (not shown) serving as the above air flow dividing device are connected in series to have the sum of their voltage outputs input to a controller 43 which in turn compares this sum with a voltage output of reference voltage circuit 44, thereby obtaining a signal to control an actuator 45 whereby the air-mix damper is moved as required. This damper is interlocked with a shaft 46a and also with a change-over rod 46 fixed for pivotal movement to the shaft 46a. The change-over rod 46 is provided at its one end with a contact piece 46b imparting an output voltage of the potentiometer 41 and at its another end with a changeover piece 46c selecting through a speed-change mechanism 49 the rotational speed of a driving motor 47 of an air supply fan (not shown) serving as the aforesaid air flow control device. By virtue of the pivotal movement of the change-over rod 46, the sum of voltage outputs of the sensors 40, 41 and the potentiometer 42, i.e., a voltage obtained by sharing a voltage difference from a power line +B to the earth E between the sum of resistances of the elements 40, 41, 42 and a temperature-setting variable resistance 48 is balanced with the above reference voltage, thereby positioning the air-mix damper and at the same time providing a corresponding driving voltage to the motor 47 to thus control the speed of rotation thereof.
In the conventional automatic temperature control system, however, the input from the outside is fed as the sum of the voltage output of the room temperature sensor and that of the outside air temperature sensor, so that, even when there is such an extent of changes in the outside air temperature as requires no operation of the air supply fan since the interior of the vehicle is at a substantially suitable temperature, the air supply fan responds to each of those changes. In other words, the operation of the air supply fan becomes more frequent, which is undesirable from the standpoint of noise control.
Moreover, in the conventional system, since the air flow dividing device and the speed-change mechanism of the motor are interlocked with each other, the supply fan is operated at every control of the air flow dividing device, thus resulting in a further increase in noise.
Furthermore, in the conventional system, the sum of the voltage output of the room temperature sensor, that of the outside air temperature sensor, and that of the potentiometer representing the flow-dividing ratio is compared with the reference voltage, so that minor errors in the actual operation of each detection circuit overlap in the comparison with the reference voltage, which is undesirable from the standpoint of circuit stability.